Process for the preparation of a dynamically vulcanized thermoplastic elastomer

ABSTRACT

The present invention relates to a process for the preparation of a dynamically vulcanized thermoplastic elastomer comprising simultaneously mixing a thermoplastic polyolefin, a vulcanizable rubber and a curing agent in a co-rotating twin-screw extruder whereby the vulcanizable rubber is cured during mixing. The residence time in the extruder is less than 40 sec, the ratio of specific Energy (Espec)/residence time is at least 24 kW/kg and the ratio of the residence time/(Length/Diameter) is less than 0.60 sec. The invention further relates to the dynamically vulcanized thermoplastic elastomer obtainable by the process and to the use of the dynamically vulcanized thermoplastic elastomer in sealing systems, building profiles and extrusion applications.

CROSS REFERENCE TO RELATED APPLICATION

This application is the National Phase of International ApplicationPCT/NL02/00647 filed Oct. 10, 2002 which designated the U.S., and thatInternational Application was published under PCT Article 21(2) inEnglish, which further claims the benefit of priority to U.S.provisional application Ser. No. 60/328,105, filed Oct. 11, 2001.

The present invention relates to a process for the preparation of adynamically vulcanized thermoplastic elastomer comprising melting andmixing a thermoplastic polyolefin, a vulcanizable rubber and a curingagent in a co-rotating twin-screw extruder, whereby the vulcanizablerubber is cured during mixing. The invention further relates to thedynamically vulcanized thermoplastic elastomer and to articlescomprising the dynamically vulcanized thermoplastic elastomer.

Dynamically vulcanized thermoplastic elastomers have a combination ofboth thermoplastic and elastic properties. The thermoplastic elastomersare prepared by mixing and shearing a thermoplastic polymer, avulcanizable rubber and a curing agent. The vulcanizable rubber is curedand is intimately and uniformly dispersed as a particulate phase withina continuous phase of the thermoplastic polymer.

From EP-A-107635 a process is known for the preparation of a dynamicallyvulcanized thermoplastic elastomer comprising a blend of a thermoplasticpolymer and a vulcanizable rubber which is subjected to high shearduring vulcanization. A disadvantage is that the black speck level ofthe dynamically vulcanized thermoplastic elastomer manufactured by theabove process is inadequate.

It is an object of the present invention to provide a process for thepreparation of a dynamically vulcanized thermoplastic elastomercomprising melting and mixing a thermoplastic polyolefin a vulcanizablerubber, and a curing agent in a co-rotating twin-screw extruder wherebythe vulcanizable rubber is cured during mixing, which overcomes theabove cited problem.

Surprisingly it has been found that when,

-   -   tr<40 sec    -   Espec/tr>=24 kW/kg    -   tr/(L/D)<0.60 sec        whereby    -   tr is understood to be the residence time in the extruder    -   Espec is defined as specific Energy in the extruder    -   (L/D) is understood to be Length/Diameter ratio of the screws in        the extruder.

A dynamically vulcanized thermoplastic elastomer has been produced witha lower black speck level. A further advantage is that the dynamicallyvulcanized thermoplastic elastomer shows a less yellow colour and has abetter odour. An additional advantage is that the dynamically vulcanizedthermoplastic elastomer has a better extrusion quality, which means lesssurface defects, a better surface roughness, surface smoothness andgloss at high extrusion rates. It is moreover surprising that by usingthe process of the present invention a dynamically vulcanizedthermoplastic elastomer can be produced with better elastic properties.

From EP-A-72203 a process is known for the preparation of a partiallydynamically vulcanized thermoplastic elastomer which comprises meltingand mixing a peroxide curable olefin copolymer rubber, aperoxide-decomposing polyolefin resin and an organic peroxide compoundin a twin screw extruder in which the dynamic heat treatment is carriedout under the condition: x<=200 wherein x stands for weight of thecopolymer rubber (g/100 particle) and y>=0.003.x+0.12 and y stands forspecific Energy at the extrusion (kWhr/kg).

A disadvantage of this process is that rubber particles with a specificdimension have to be used.

Any co-rotating twin screw extruder capable of generating a ratio ofspecific Energy/residence time of at least 24 kW/kg and a ratio of theresidence time/(Length/Diameter) of less than 0.60 sec is usefull forcarrying out the process of the present invention. Specific Energy(Espec) is here and hereafter defined as the mechanical energy in kWhsupplied by the motor of the extruder to the thermoplastic elastomer perkg (kW.h/kg). The specific Energy is for example at least 0.20 kWh/kg.Preferably, the specific Energy is at least 0.24 KWh/kg, more preferablythe specific Energy is at least 0.28 kWh/kg. The residence time (tr) isthe time period between the dosing of few pieces of contrasting colorgranulate together with the thermoplastic polyolefine, the vulcanizablerubber and the curing agent in the entry port of the extruder and thefirst moment that a change in color at the die exit is noticable. Theresidence time in the extruder is preferably less than 35 sec, morepreferably less than 30 sec, most preferably less than 25 sec.

Twin-screw extruders, suitable in the process of the present inventionhave screws which may vary in diameter from 25 mm to 200 mm, the ratioof Length of the screws over the Diameter of the screws (UD) may varyfrom 16-60, preferably from 38-60, more preferably from 42-56.

The rotation speed of the screws may vary from 500-1200 revolutions perminute (rpm), but is preferably at least 700 rpm, more preferably atleast 900 rpm, most preferably 1100 rpm.

An extruder satisfactory for carrying out the process of the presentinvention is for example a mega-compounder extruder produced by Werner&Pfleiderer™, Germany or an Ultra-torque (UT) Berstroff extruder.

Examples of suitable thermoplastic polyolefins used in the dynamicallyvulcanized thermoplastic elastomer according to the present inventionare thermoplastic polyolefin homo- and copolymers or blends thereof. Forexample homopolymers of ethylene or propylene, copolymers of ethyleneand propylene, copolymers of ethylene and an alpha-olefin with 4-12carbon atoms or copolymers of propylene and an alpha-olefin with 4-12carbon atoms may be used. Preferably, a homopolymer of propylene isused.

Examples of vulcanizable rubbers that may be used in the dynamicallyvulcanized thermoplastic elastomer according to the present inventionare ethylene-propylene copolymers, hereinafter called EPM,ethylene-propylene-diene terpolymers, hereinafter called EPDM, styrenebutadiene rubber, nitrile butadiene rubber, isobutene-isoprene rubber,styrene-ethylene/styrene-butadiene block copolymers, butyl rubber,isobutylene-p-methylstyrene copolymers or brominatedisobutylene-p-methylstyrene copolymers, natural rubber or blends ofthese.

Preferably, EPDM or EPM is used as vulcanizable rubber. Most preferably,EPDM is used as vulcanizable rubber. The EPDM preferably contains 50-70parts by weight ethylene monomer units, 48-30 parts by weight monomerunits originating from an alpha-olefin and 2-12 parts by weight monomerunits originating from a non-conjugated diene. As alpha-olefin use ispreferably made of propylene. As non-conjugated diene use is preferablymade of dicyclopentadiene (DCPD), 5-ethylidene-2-norbornene (ENB) orvinylnorbornene (VNB).

The vulcanizable rubber is at least partially vulcanized. Thoseordinarily skilled in the art will appropriate quantities, types ofcuring agents and vulcanization conditions required to carry out thevulcanization of the rubber.

The vulcanizable rubber can be vulcanized using varying amounts ofcuring agent, varying temperatures and varying time of vulcanization aslong as the conditions of the process according to the present inventionare fullfilled. Examples of curing agents which may be used in thepreparation of the dynamically vulcanized thermoplastic elastomeraccording to the present invention are sulfur, sulfurous compounds,metal oxides, maleimides, phenol resins or peroxides with or withoutaccelerators and/or co-agents. Curing agents, accelerators and co-agentsare known to the skilled person and are for example described in U.S.Pat. No. 5,100,947. It is also possible to use siloxane compounds ascuring agent, for example hydrosilane or vinylalkoxysilane. Thevulcanizable rubber is preferably vulcanized with the aid of phenolresins, siloxanes or peroxides as curing agent.

The invention also relates to the dynamically vulcanized thermoplasticelastomer obtainable by the process of the present invention. The degreeof vulcanization of the dynamically vulcanized thermoplastic elastomercan be expressed in terms of gel content or conversely, extractablecomponents. Gel content reported as percent gel is determined by aprocedure which comprises determining the amount of insoluble polymer bysoaking a specimen of the thermoplastic elastomer for 48 hours inorganic solvent at room temperature and then weighting the dried residueand making suitable corrections based upon knowledge of the composition.Thus corrected initial and final weights are obtained by substractingfrom the initial weight, the weight of soluble components, other thanthe rubber to be vulcanized, for example extender oils, plasticizers andcomponents of the compositions soluble in organic solvent. Theexpression is well known in the art and described in U.S. Pat. No.5,100,947.

The vulcanized rubber in the dynamically vulcanized thermoplasticelastomer according to the present invention is at least partlyvulcanized and the gel content may vary between 60 and 100%. Preferably,the gel content of the vulcanized rubber is in excess of 80%. Morepreferably, the vulcanized rubber is fully vulcanized and the gelcontent is in excess of 95%.

The dynamically vulcanized thermoplastic elastomer according to thepresent invention may, for example, comprise between 5-35% by weight ofthe thermoplastic polyolefin and between 65-95% by weight of thevulcanizable rubber based on the weight of the dynamically vulcanizedthermoplastic elastomer. Preferably, the dynamically vulcanizedthermoplastic elastomers comprises between 7-25% by weight of thethermoplastic polyolefin and between 75-93% by weight of thevulcanizable rubber based on the weight of the dynamically vulcanizedthermoplastic elastomer.

The dynamically vulcanized thermoplastic elastomer according to thepresent invention optionally comprises other customary additives whichmay be compounded. Examples of such additives are reinforcing andnon-reinforcing fillers, plasticizers, antioxidants, rubber processingoil, extender oils, lubricants, antiblocking agents, antistatic agents,waxes, foaming agents, pigments, flame retardants and other processingaids known in the rubber compounding art. Examples of fillers that maybe used are calcium carbonate, clays, silica, talc, titanium dioxide,carbon or mixtures of any of these. Examples of rubber processing oilsthat may be used are paraffinic, naphthenic or aromatic oil derived frompetroleum fractions or mixtures of any of these.

In the process of the present invention the thermoplastic polyolefine,the vulcanizable rubber, the curing agent and the customary additivesmay, all and in whole, be fed into the entry port of the extruder. It isalso possible that one or more of the thermoplastic polyolefine, thevulcanizable rubber, the curing agent or the additives are fed in wholeor in part at a place downstream in the extruder. If the thermoplasticpolyolefine, the vulcanizable rubber, the curing agent and the customaryadditives are fed at different places, the vulcanizable rubber and apart of the thermoplastic polyolefine are dosed in the entry port of theextruder together with the additives and mixed. After the vulcanizablerubber, the thermoplastic polyolefine and the additives have beenproperly dispersed, the curing agent and additional thermoplasticpolyolefine may be added. It is also possible to add additional extenderoil, plasticizer, colorants, thermoplastic polyolefine or curing agentat another entry port located in about the middle of the extruder.

The present invention further relates to the use of the dynamicallyvulcanized thermoplastic elastomer in sealing systems, building profilesor extrusion applications.

The present invention also relates to articles comprising thedynamically vulcanized thermoplastic elastomer. These articles may beapplied in cars, equipment and constructions. Examples hereof are airbagcovers, fuel lines, hoses, dashboard foils, spoilers, mud flaps, seals,boots and strips.

The invention will be elucidated by means of the following examples andcomparative experiments without being limited thereto.

All dynamic vulcanizations were executed on a co-rotating Werner &Pfleiderer™ twin screw extruder. The L/D ratio of the screws in theextruder was 52 (Examples I, II, III and comparative experiment A). TheL/D ratio in comparative experiment B was 44. The throughput was varyingfrom 850-3700 kg/hr at screw speeds varying from 500-1200 rpm. The melttemperature at the end of the extruder was between 260-300° C. All theindicated raw materials were added on a hopper except 40 parts of oilwhich were injected in the extruder, after the dynamic vulcanization hadbeen started.

The properties of the dynamically vulcanized thermoplastic elastomerswere analysed on injection moulded parts.

Different properties were analysed;

-   -   Hardness, Shore A, according to ASTM D-2240-91    -   Tensile strength, Mpa, according to ASTM D-412-92, Method A, Die        C    -   Elongation, %, according to ASTM D-412-92, Method A, Die C    -   100% modulus, Mpa, according to ASTM D-412-92, Method A, Die C    -   Compression set (CS), %, according to ASTM D-395-89, Method B    -   Density, according to ASTM D-792-91    -   Colour b-value was measured using a BYK colorgard 9000,        calorimeter. It uses a 45 degree circumferential/0 degree        viewing geometry. The diffuse reflectance from the surface is        measured by filling a cup with pellets and the measurements        taken when the instrument looks up into the cup from the bottom        surface. It measures 5 times and averages those results to give        one test reading.    -   Odour is measured according to VDA 270.    -   Extrusion quality was measured visual by 3 persons who defined        the surface defects, the surface roughness, surface smoothness        and gloss and is expressed as the average of the 3 assesments by        a score between 1-5. 1=bad extrusion quality, 4=acceptable        extrusion quality for customers, 5=excellent extrusion quality.    -   Black speck level was measured on 5 injection moulded parts of        8×8 cm, with a thickness of 2 mm from which the contaminants        (black spots) were counted by using an ocular. The contaminants        are black spots which are bigger than 0.6 mm and those which are        smaller than 0.6 mm. After measuring, the average number of        black spots bigger than 0.6 mm and the average number of black        spots smaller than 0.6 mm is calculated.    -   Residence time in the extruder (tr) is the time period measured        between the dosing of few pieces of contrasting color granulate        together with the thermoplastic polyolefine, the vulcanizable        rubber and the curing agent in the entry port of the extruder        and the first moment that a change in color at the die exit was        noticable.    -   Specific Energy (Espec) is the mechanical energy in kWh supplied        by the motor of the extruder to the thermoplastic elastomer per        kg (kW.h/kg) as described in Plastic compounding by DB Todd ISBN        1-56990-236-4.

EXAMPLE I

A mixture of 200 parts of EPDM (Keltan P597™), 33 parts of polypropylenehomopolymer with a melt flow index of 1.2 g/10 min, 2 parts stannouschloride, 5 parts phenolic resin (Schenectady SP 1045™), 2 parts zincoxide and 40 parts talc was dynamically vulcanized at a screw speed of800 rpm. Additionally 40 parts of oil (Sunpar 150™) was added at themiddle of the extruder via injection. The residence time in the extruderwas 29 sec, the ratio residence time/(L/D) was 0.55 sec and the ratioEspec/residence time was 39.7 kW/kg. Properties of the dynamicallyvulcanized thermoplastic elastomers are given in table 1.

EXAMPLE II

Same procedure and composition as in example I but the residence timewas 22 sec, the ratio residence time/(L/D) was 0.42 sec and the ratioEspec/residence time was 52.3 kW/kg. Properties of the dynamicallyvulcanized thermoplastic elastomers are given in table 1.

EXAMPLE III

Same procedure and composition as in example I but the residence timewas 18 sec, the ratio residence time/(L/D) was 0.35 sec and the ratioEspec/residence time was 66 kW/kg. Properties of the dynamicallyvulcanized thermoplastic elastomers are given in table 1.

Comparative Experiment A

Same procedure and composition as in example I but the residence timewas 44 sec, the ratio residence time/(L/D) was 0.85 sec and the ratioEspec/residence time was 22.9 kW/kg. Properties of the dynamicallyvulcanized thermoplastic elastomers are given in table 1.

Comparative Experiment B

Same composition as in example I but the L/D was 44, the residence timewas 50 sec, the ratio residence time/(L/D) was 1.14 sec and the ratioEspec/residence time was 20.88 kW/kg. Properties of the dynamicallyvulcanised thermoplastic elastomers are given in table 1.

TABLE I Properties of the dynamically vulcanized thermoplasticelastomers Example/ experiment I II III A B Screw L/D 52 52 52 52 44Throughput 990 1320 1650 660 400 (lb/hr) Screw speed 800 1000 1200 500500 (rpm) Espec 0.32 0.32 0.33 0.28 0.29 (kW h/kg) Residence 29 22 18 4450 time (tr) (sec) tr/(L/D) (sec) 0.55 0.42 0.35 0.85 1.14 Espec/tr 39.752.3 66.0 22.9 20.8 (kW/kg) Hardness 62.7 61.5 62.1 62.6 62.4 (shore A)Density 0.978 0.984 0.982 0.972 0.981 (g/cm³) Tensile 5.1 5.6 5.9 5.65.5 Strenght (Mpa) 100% 2.0 2.0 2.0 2.0 1.9 modulus (Mpa) Elongation at435 425 455 418 491 break (%) Compression 21.6 21.5 21.3 23.0 24.0 set(22 hr/ 70° C.) Black speck 77 69 58 95 109 level Colour 22.5 21.1 19.923.7 22.7 b-value Extrusion 4.25 4.75 4.5 4 2 quality (1-5) Odour 2.52.5 1.5 2 1.5

As can be seen from the examples I, II and III and comparativeexperiments A and B in table 1, dynamically vulcanized thermoplasticelastomers have been produced which have a much better black speck levelif the residence time in the extruder is less than 40 sec, the ratio ofspecific Energy (Espec)/residence time is at least 24 kW/kg and theratio of the residence time/(Length/Diameter) is less than 0.60 sec.Also a better colour and extrusion quality has been achieved. Moreover abetter compression set has been received under the above conditions.

1. Process for the preparation of a dynamically vulcanized thermoplasticelastomer comprising melting and mixing a thermoplastic polyolefin, avulcanizable rubber and a curing agent in a co-rotating twin-screwextruder, whereby the vulcanizable rubber is cured during mixing,wherein tr<40 sec, Espec/tr>=24 kW/kg, tr/(L/D)<0.60 sec, and therotation speed of each co-rotating twin screw in the extruder variesfrom 500 to 1200 rpm; and whereby tr means the total residence time inthe extruder, Espec means specific Energy (kWh/kg) in the extruder andrepresents mechanical energy in kWh supplied by an extruder motor to thethermoplastic elastomer per kg, and (L/D) means Length/Diameter ratio ofthe screws in the extruder.
 2. Process according to claim 1, wherein theresidence time in the extruder is less than 30 sec.
 3. Process accordingto claim 1, wherein the screw speed is at least 700 rpm.
 4. Processaccording to claim 1, wherein the screw speed is at least 900 rpm. 5.Process according to claim 1, wherein the thermoplastic polyolefin is ahomopolymer of ethylene or propylene or a copolymer of ethylene andpropylene.
 6. Process according to claim 1, wherein the vulcanizablerubber is EPDM or EPM.
 7. Process according to claim 1, wherein therubber is vulcanized to a gel content in excess of 80%.
 8. Processaccording to claim 7, wherein the rubber is vulcanized to a gel contentin excess of 95%.
 9. Process according to claim 1, wherein thedynamically vulcanized thermoplastic elastomer comprises between 5-35 wt% of the thermoplastic polyolefin.
 10. Process according to claim 1,wherein the dynamically vulcanized thermoplastic elastomer comprisesbetween 95-65 wt % of the vulcanizable rubber.
 11. Dynamicallyvulcanized thermoplastic elastomer obtainable by the process accordingto claim
 1. 12. Article comprising the dynamically vulcanizedthermoplastic elastomer according to claim
 11. 13. Article according toclaim 12 in sealing systems, building profiles or extrusionapplications.